A notebook personal computer (hereinafter, referred to as a notebook PC) available in a mobile environment is typically equipped with a charging-type battery. In an environment in which a commercial power supply can be used (i.e., in an office or at home), a battery is charged while electric power is supplied to a load by using an AC/DC adapter connected to the commercial power supply. In an environment in which a commercial power supply cannot be used, electric power is supplied by switching to a battery feed so as not to interrupt power supplied to a load.
Japanese Unexamined Patent Publication (Kokai) No. 2004-304976 provides a disclosure of a power supply system which supplies electric power from an AC adapter to a circuit in a set while charging one piece of battery. Japanese Unexamined Patent Publication (Kokai) No. 2000-32682 provides a disclosure of a power supply system which supplies electric power from DC-IN to an electric power consumption device while charging two pieces of batteries.
FIG. 1 is a block diagram of a power supply system generally adopted in a notebook PC. An AC/DC adapter 10 converts an AC voltage to a DC voltage, and supplies electric power to a load 14 according to a path 11. An FET 1 and either an FET 8 or an FET 9 are set to be ON while charging. In addition, an FET 5 or an FET 7 is set to be ON corresponding to the FET 8 or the FET 9. An FET controller 17, an FET 2 and an FET 3 configure a battery charger. The FET controller 17 performs switching control of the FET 2 and the FET 3 in a synchronous rectification method, and converts the DC voltage generated by the AC/DC adapter 10 to a DC voltage suitable for charging. Electric power generated by the AC/DC adapter 10 is supplied to the load 14 while charging according to the path 11, and is supplied to a battery 15 or a battery 16 for charging according to a path 12 or a path 13. When the power feed is switched from a commercial power supply to a battery, the FET 1 is set to be OFF, the FET 8 or the FET 9 corresponding to the battery being charged is set to be OFF, and an FET 4 or an FET 6 is set to be ON. Electric power is supplied from a battery while charging to the load 14 according to a path 18 or a path 19. Nine FETs are used in the power supply system shown in FIG. 1.
FIG. 2 is a block diagram of another power supply system which has also conventionally been adopted in a notebook PC. In FIG. 2, the same reference numerals are attached to the same elements as FIG. 1. In the power supply system in FIG. 2, there are eight FETs, which is one fewer than the number of FETs used in the power supply system of FIG. 1. From the power supply system in FIG. 2, the FET 8 and the FET 9 in FIG. 1 are removed and an FET 20 is provided therein in order to configure a discharge path for flowing an electric current from the battery to the load 14. The FET 20 is set to be ON while discharging to pass discharge current, and is set to be OFF while charging so that the voltage of the battery charger can be applied to the battery effectively.
In view of the foregoing, a need has been recognized in connection with improving upon the shortcomings and difficulties of conventional efforts to reduce the reduced number of switching elements in a power supply while maintaining the function of the power supply.